Apparatus and method for firing magnetic cores

ABSTRACT

A kiln for sintering small magnetic cores or other parts, which includes a vertically extending tube that is heated along a center section. Magnetic cores of a diameter much smaller than the inside diameter of the tube, drop in free fall through the tube and emerge fully heat treated at the lower end. An additional inlet at the upper end is connected to a pressurized gas source to pump air, oxygen, or other gas at a controlled rate through the tube to control the speed at which the cores move through the tube. An additional gas inlet is connected immediately above the lower end of the tube for receiving cooling gases to cool the cores in a controlled atmosphere prior to emergence at the lower end of the tube.

[1 1 3,733,i7i 1 May15,1973

8/196l Albers-Schoenberg................263/4l John J. Camby erg,Freilich and Wasserman ABSTRACT gnetic cores or other g tube that n. f ma e d n a e 10 U t e h t 1 DO gh the tube. An additional y above thelower g gases to cool the phere prior to emergence 11 Claims, 5 DrawingFigures Primary Examiner- A ttrney- Lindenb A kiln for sintering smallma parts, which includes a vertically extendin is heated along a centersection. Magnetic cores of a diameter much smaller than the insidediamet tube, drop in free fall throu fully heat treated at the lowe atthe upper end is connected to a source to pump air, oxygen,

trolled rate through the tube to contro which the cores move throu gasinlet is connected immediate] end of the tube for receiving coolin coresin a controlled atmos at the lower end of the tube.

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[54] APPARATUS AND METHOD FQIR FIRING MAGNETIC QORES Inventors: Robert,ll. Sisolak, Edison, N.J.; Jack 111. Bryant, Palos Verdes PeninsulaCalif.

[22] Filed:

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W d td HUN, 1]] 218 555 [ill [5 6] References Cited UNITED STATESPATENTS 2,806,783 Tschop et 1,132,738 Schwahn..............

APPARATUS AND' METHOD FOR FIRING MAGNETIC CORES BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates to heattreating apparatus and methods.

2. Description of the Prior Art Small magnetic cores are used in greatvolume in computer memories and other applications. One method used fortheir production includes pressing powdered metal into a toroidal orother shape to obtain a green core, and then heating the core in aclosely controlled manner to sinter it so that it obtains the desiredmechanical and electrical properties. In one major type of furnacearrangement which has been used for mass production sintering, the coresare placed on ceramic boats or saggers that are slowly moved through afurnace. It often requires over one hour for a core to progress throughthe heat treatment cycle, leading to low production rates and difficultyin maintaining close control by testing finished cores and altering thefurnace controls accordingly. In addition, the atmosphere in the furnacecannot be controlled too closely, since air can enter every time doorsare opened to admit or release a sagger from the furnace.

Another method which has been used for heat treating the cores involvesthe use of a platinum belt that extends through a furnace to carryindividual cores thereon. While this apparatus enables a more rapidcycle time such as one or two minutes, it still has many disadvantages.A cycle time of l or 2 minutes still means that a large number of coresmay be improperly sintered before the furnace settings are altered, evenwhere samples are quickly tested at the output end of the belt. Anotherdisadvantage is that uncontrolled amounts of air can leak into thefurnace through the openings where the belt enters and exits, and thatcores lying near the center of the belt may be heat treated differentlythan those near the edge of the belt, so that close control of sinteringis difficult. A further problem is that the platinum belt wears andages, and its cost of replacement is high. Furthermore, cores mayoccasionally stick to the belt or fall off the belt inside the kiln and,when they finally drop off into the collecting box after undergoingnumerous heat treat cycles, they may be misshapen and tend to clog thetesting apparatus in addition to having unsatisfactory electricalproperties.

OBJECTS AND SUMMARY OF THE INVENTION An object of the invention is toprovide-heat treating apparatus for small parts, that can beconstructed, operated, and maintained at relatively low cost.

Another object is to provide heat treating apparatus for small partsthat'enables very close control of the heating cycle even atvery highproduction rates.

Yet another object is to provide a method for the heat treating of smallparts, which is rapid and economical, and which enables very closecontrol.

In accordance with one embodiment of the present invention, an apparatusand method is provided for the economical and high precision heattreating of small parts, and particularly for the sintering of magneticcores. The apparatus includes a tube of platinum or other hightemperature-resistant material that extends substantially verticallythrough a furnace. Parts are dropped into a portion of the tube abovethe furnace and collected as they emerge from a location below thefurnace. In order to maintain a closely controlled atmosphere at theheated portion of the tube, a pressured gas is applied to the upper endof the tube, and is regulated to flow at a controlled rate through thetube. The flow rate is adjusted so that the cores fall through the tubeat a controlled rate, to thereby control the period during which eachcore is maintained in the heated region of the tube. Another gas inletis provided near the bottom of the tube, below the central heatedportion but above the exit where heat treated parts emerge. A coolinggas is admitted into this inlet to provide a controlled atmosphere forcooling the cores before they drop into the collecting box.

The heat treating apparatus can be constructed at relatively low cost,inasmuch as there are no moving parts in the high temperature zone ofthe furnace, except for the parts being heat treated. The magnetic corescan be sintered in a short time, such as several seconds, so that highproduction and close control can be achieved. The atmosphere in the tubecan be closely controlled to provide highly uniform results.

The novel features of the invention are set forth with particularity inthe appended claims. The invention will be best understood from thefollowing description when read in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional side view ofheat treating apparatus constructed in accordance with one embodiment ofthe present invention;

FIG. 2 is a view taken on the line 2-2 of FIG. 1; FIG. 3 is an enlargedview of the region 3-3 of FIG.

FIG. 4 is an enlarged view of the region 44 of FIG. 2; and

FIG. 5 is an enlarged section view of a portion of the tube of FIG. 1,showing the manner in which magnetic cores fall therethrough.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates heat treatingapparatus, including a kiln 12, a parts feeder 14 for feeding smallmagnetic cores or other small parts into the kiln, and a box 16 forcollecting the cores after they pass through the kiln. The kiln includesa vertically-extending carry tube 18, a vertically-extending shroud tubedisposed about the carry tube, and numerous heating elements 22 disposedalong most of the length of the tubes. A firebrick insulation column 24surrounds the tubes and the center portions of the heating elements, anda steel frame 26 encases the firebrick column. The parts feeder 14 isnormally filled with green, or unsintered, magnetic cores, and it dropsthem into an upper portion 28 of the carry tube for free fall throughthe tube. During such free fall, the cores are exposed to intense heat,inasmuch as a long center portion of the carry tube 18 is heated. Whenthe cores emerge at a lowerportion 30 of the tube, they have been fullyheat treated and are ready to be tested and thereafter installed inelectronic equipment.

If the cores were merely dropped into the carry tube 18, whose lowerportion 30 is open, the cores would not fall through the tube. This isbecause the intensly heated carry tube 18 acts like a chimney to createa strong updraft therethrough. In order to prevent such a strongupdraft, gas from a pressured gas source 32 is connected through avolume flow regulator 34 to the upper portion 28 of the carry tube 18 topump gas downwardly through the tube. This allows cores fed into thetube by the parts feeder 14 to fall therethrough and exit at the lowerend 30 into the box 16. The flow regulator 34 not only allows the coresto fall down through the tube, but enables a relatively close control ofthe rate at which gas flows through the carry tube 18. A control of theflow rate of gas results in control of the rate at which the cores fallthrough the tube. This is because the cores are amall, so that they havea high air resistance-to-weight ratio. Accordingly, their free fallspeed can be readily controlled by controlling the rate of movement ofthe atmosphere through which they fall. Another gas source 36 is coupledto the lower portion 30 of the carry tube below the heated centralportion thereof, to introduce a cooling gas. The cooling gas helps tocool the cores prior to their emergence at the extreme lower end of thecarry tube into the box 16.

The gas sources 32 and 36 that are coupled to the carry tube 18 not onlyserve to control the rate of descent of cores through the tube and tocool the cores, but also enable close control of the atmosphere duringthe process. That is, these gas sources enable a selection of the typeof atmosphere that surrounds the cores during heating and cooling.Magnetic cores may be constructed of a variety of materials, and eachmay require a different atmosphere during heat treating. For example,magnesium-manganese-ferrite cores require an oxidizing atmosphere duringfiring, but a neutralizing atmosphere during cooling. For some cores ofthis construction, the pressured gas source 32 that is connected to theupper end of the carry tube may supply oxygen mixed in a predeterminedratio with other gas components, while the gas source 36 connected tothe lower end of the tube may be nitrogen. In many cases, the gases maybe air, so that the pressured gas source 32 is a pump and the lower gassource 36 is merely an opening into the ambient atmosphere.

FIG. 4 illustrates the region where the lower portion 30 of the carrytube 18 is connected to a cooling feed tube 38 that carries the coolinggas from the gas source 36. The cooling feed tube 38 is coupled to thecarry tube 18 to extend with a downstream directional component, so thata partial pressure is created in the feed tube 38. This allows air fromthe ambient atmosphere to be sucked into the feed tube 38 where the feedtube is merely open to the atmosphere instead of being connected to apressurized gas source.

It is generally necessary that the carry tube 18 extend in a primarilyvertical direction. Tests have been conducted using a carry tubeoriented horizontally and with air pressure applied to one end to blowsmall parts therethrough. However, it was found to be extremelydifficult to closely control the rate at which the parts move throughthe tube, particularly if it is desired to move them slowly through thetube. It is desirable to orient the carry tube so that it extendsprimarily in a vertical direction rather than a horizontal direction, atleast along the center portion that is heated. Satisfactory kilns havebeen constructed where the carry tube extends within several degrees ofthe vertical.

It may be noted that the tendency to a strong updraft through the carrytube 18 can be prevented by blocking the lower portion 30 of the carrytube to the ambient atmosphere, as by connecting the lower portion 30 toa sealed box 16. Cores have been dropped into such apparatus, and theycan fall through the carry tube in that case. However, the atmosphere inthe carry tube quickly becomes unsatisfactory, largely due to thedepletion of oxygen by the cores. Thus, after a limited number of coreshave been fired the process becomes unsatisfactory, even in the case ofcores that can be fired and cooled in air. It may also be noted thatsome air can leak into the upper end of the carry tube through a partsfeed tube 40 that connects the parts feeder 14 to the carry tube 18.Where such leakage is considered a problem, the parts feeder 14 can beenclosed in an airtight case to prevent air leakage. Typically, avibratory parts feeder may be employed which is of small size, so thatsuch encasing is easily accomplished.

The kiln 12 is constructed to define four different heating zones 41,42, 43, and 44 along the central length of the carry and shroud tubes18, 20. Three baffles 48 between the firebrick column 24 and shroud tube20 seal the heating zones from one another. Four thermocouples 50connected to meters (not shown) measure the temperature of the shroudtube 20 near the top of each heating zone. The heating elements 22 canbe silicon carbide rods of the type which have a high electricalresistance at the center portions thereof which lie near the shroud tube20. The temperature of each zone can be closely controlled by regulatingthe current delivered to the heating elements within the zone. Theprovision of several heating zones enables more complex heat treatingprocesses to be carried out.

The shroud tube 20 is constructed of a high heatresistant material suchas alumina. The carry tube 18 is preferably constructed of aheat-resistant material which is highly resistant to corrosion, such asplatinum. The cost of a platinum tube is minimized because the carrytube 18 is of small diameter. The heat treating of the magnetic cores istypically carried out at temperatures such as 2,300 to 2,400 F, andsometimes at a temperature several hundred degrees higher. At thesetemperatures, large amounts of heat can be transferred by radiation.Accordingly, the heating elements 22 heat the shroud tube 20, the shroudtube 20 heats the carry tube 18, and the carry tube 18 heats the corespassing therethrough, all largely by radiation. Although air currentsare created in the space between the shroud tube 20 and carry tube 18, arapid updraft is prevented by the use of a cap 52 at the upper portionof the tubes to seal the space between them. The convection air currentsbetween the tubes 18, 20 is small enough so that separately controllabletemperatures can be maintained in the different temperature zones 41,42, 43, and 44. An additional cap is not required at the lower end ofthe tubes, although a spider or other means can be employed to maintainthe tubes 18, 20 spaced from one another.

FIG. 5 illustrates the manner in which magnetic cores 54 fall throughthe carry tube 18. The great heating of the atmosphere flowing throughthe carry tube creates air currents and these plus the variable airresistance of the tumbling cores causes the cores to move in a randommanner to different positions within the tube, sometimes bouncing gentlyofi the walls of the tube. During the period of descent, all surfaces ofthe cores are exposed to heat, both by radiation from the heated tube18, and by conduction from the heated atmosphere surrounding the cores.Thus, all cores are heat treated nearly identically, and the heattreatment is applied to all regions of the cores. It can be seen thatthe inside diameter D, of the tube is much larger than the diameter D,of the cores. This allows the cores to tumble freely and to bounce offthe walls of the tube 18 only occasionally. It is generally desirablethat the diameter D, of the tube be at least four times as great as thelargest linear dimension of the parts passing therethrough.

Heat treating apparatus has been constructed utilizing a kiln having alength L of about eight feet and employing a platinum carry tube 18 withan inside diameter of about one-fourth inch. Cores of a diameter such asone-fifieth inch were heat treated by passing them through the kiln. Therate of gas flow through the carry tube was regulated so that itrequired about three seconds for the cores to pass through the heatedportion, of about 7 feet in length, of the carry tube. This is aboutfour times as long as it would require for an object to fall the samedistance in a vacuum. Thus, a kiln of moderate length can be employed toheat treat free falling parts, even where the heat treating is to becarried out over a period of a few seconds. Of course, a few seconds isa relatively short period of time, as compared to prior methods. Thisshort period is possible because the cores are free of the heat-sinkeffect of a belt or the like on which they have been carried in priorapparatus, and the small size of the cores allows them to be heated veryrapidly.

Core testing equipment can be provided to test samples of cores soonafter they emerge from the lower portion 30 of the carry tube. Theresults of the sample test can be utilized to alter the temperatures inthe kiln, the flow rate through the flow regulator 34, or otherconditions. The fact that the cycle time during which the cores areheated is short, often being only a few seconds, means that correctionscan be made rapidly and before too many additional cores have been heattreated, for a given production rate. The small size of the apparatusalso enables conditions to be changed rapidly. The heat treatingapparatus of this invention has numerous advantages over the apparatusheretofor conventionally used in the heat treatment of small parts, andparticularly magnetic cores, wherein the cores were carried on belts orsaggers through a furance. The short cycle term enables very highproduction rates, such as. several million cores per hour, even with therelatively small kiln described above. Samples of the cores can becycled and tested, and adjustments can be quickly made in the apparatus.Closely uniform heat treating is applied to all of the cores and to thedifferent parts of each core. This may be compared with apparatuswherein cores are carried on belts or the like, and wherein cores nearthe edges of the belt may be heated differently from those near themiddle, and where the lower part of the core that rests on the belt maynot be heated in the same manner as the opposite side which is directlyexposed to the heat. The apparatus of this invention enables closecontrol in a simple manner, of the atmospheres in which the cores areheated and cooled. The apparatus has no parts moving within the hightemperature portion of the kiln, except for the cores themselves, sothat construction and upkeep is minimized.

Thus, the invention provides a relatively simple apparatusand method forthe heat treating of parts, and particularly the sintering of magneticcores, by the use of a tube with a heated central portion for carryingthe cores in substantially free fall. The rate of fall of the cores aswell as the atmosphere in which they are heated, is controlled bycontrolling the admittance of a pressurized gas into an upper endportion of the tube. Cooling of the cores prior to their falling into acollecting box or the like is encouraged by coupling a tube to the lowerportion of the carry tube at a location above the core exit, to admitcooling gases. The cooling tube may be exposed to the ambientatmosphere, or may be coupled to a supply of a desired gas, and thepressure or flow rate of cooling gas can also be controlled.

Although particular embodiments of the invention have been described andillustrated herein, it is recognized that modifications and variationsmay readily occur to those skilled in the art and, consequently, it isintended that the claims be interpreted to cover such modifications andequivalents.

What is claimed is: 1. Apparatus for heat treating small partscomprising: an elongated tube having first and second end portions andhaving a parts inlet and a parts outlet at said first and second endportions, respectively; means for heating a center portion of said tube;means for supporting said tube to extend in a primarily verticaldirection, with said first end portion above said second end portion;and means coupled to said first end portion of said tube for supplyingpressured gas to flow through at least said center portion of said tube,including a source of pressured gas and a volume flow regulator couplingsaid source to said tube to control the volume rate at which gas flowsinto said tube, whereby to control the time of passage of said partsthrough said heated center portion. 2. A method for heat treating smallparts comprising: dropping said parts into a primarily verticallyextending tube; heating at least a predetermined portion of said tube;

and introducing a cooling gas through a lower portion of said tube tomove with said parts along a portion of said tube. 3..A method for heattreating small parts comprising: dropping said parts into a primarilyvertically extend ing tube; heating at least a predetermined portion ofsaid tube;

and flowing gas through said predetermined portion of said tube which isheated, at a rate that causes said parts to fall at a speed therethroughso that they move through said heated portion in a time period which isat least twice the free-fall time of said parts in a vacuum environmentof the same dimensions. 4. Apparatus for heat treating small partscomprising:

an elongated tube having first and second end portions;

means for holding said tube so that it extends in a primarily verticaldirection, with said first end portion above said second end portion;

means for heating a center portion of said tube that lies between saidend portions;

parts feeding means for feeding parts individually into said first endportion of said tube, so that they fall through said tube spaced fromone another; and

means for receiving parts leaving said second end portion of said tube.

5. The apparatus described in claim 4 including:

a gas inlet coupled to said first end portion of said tube; and

means for supplying pressured gas to said gas inlet, to

create a gas flow through said tube.

6. Apparatus for heat treating small parts comprising:

an elongated tube having first and second end portions;

means for holding said tube so that it extends in a primarily verticaldirection, with said first end portion above said second end portion;

means for heating a center portion of said tube that lies between saidend portions;

means for feeding parts into said first end portion of said tube, sothat they fall through said tube;

means for receiving parts leaving said second end portion of said tube;and

means coupled to said tube between said center portion of said tube andthe location where parts leave at said second portion of said tube, forinserting a cooling gas into said tube to move with said parts as theyfall towards the location where the parts exit from the tube.

7. A method for sintering green cores comprising:

dropping said cores individually into a primarily vertically extendingtube so that they are spaced from one another during their fall throughthe tube; and

heating at least a predetermined portion of said tube.

8. The method described in claim 7 including:

applying gas to the upper end of said tube at a controlled gas flowrate, whereby to regulate the passage of small parts which have a highair resistance compared to their weight.

9. Apparatus for heat treating small parts comprising:

an elongated outer tube with a central portion;

means disposed along said central portion of said outer tube for heatingthe outside thereof;

an inner tube disposed within and extending through said outer tube, sothat said tubes are free of contact with one another along substantiallyall of said central portion of said outer tube which is heated; and

means at opposite end portions of said inner tube for receiving anddelivering said parts.

10. The apparatus described in claim 9 wherein:

said outer and inner tubes extend substantially vertically; andincluding means coupled to the upper end of said inner tube for forcinga gas downwardly therethrough.

11. The apparatus described in claim 9 wherein:

at least one end of the space between said central portion of said outertube and said inner tube is sealed against the axial flow of gastherethrough; and

said means for heating includes a plurality of separately controllableheating means spaced along said center portion of said outer tube, tocreate a plurality of separate heating zones.

1. Apparatus for heat treating small parts comprising: an elongated tubehaving first and second end portions and having a parts inlet and aparts outlet at said first and second end portions, respectively; meansfor heating a center portion of said tube; means for supporting saidtube to extend in a primarily vertical direction, with said firsT endportion above said second end portion; and means coupled to said firstend portion of said tube for supplying pressured gas to flow through atleast said center portion of said tube, including a source of pressuredgas and a volume flow regulator coupling said source to said tube tocontrol the volume rate at which gas flows into said tube, whereby tocontrol the time of passage of said parts through said heated centerportion.
 2. A method for heat treating small parts comprising: droppingsaid parts into a primarily vertically extending tube; heating at leasta predetermined portion of said tube; and introducing a cooling gasthrough a lower portion of said tube to move with said parts along aportion of said tube.
 3. A method for heat treating small partscomprising: dropping said parts into a primarily vertically extendingtube; heating at least a predetermined portion of said tube; and flowinggas through said predetermined portion of said tube which is heated, ata rate that causes said parts to fall at a speed therethrough so thatthey move through said heated portion in a time period which is at leasttwice the free-fall time of said parts in a vacuum environment of thesame dimensions.
 4. Apparatus for heat treating small parts comprising:an elongated tube having first and second end portions; means forholding said tube so that it extends in a primarily vertical direction,with said first end portion above said second end portion; means forheating a center portion of said tube that lies between said endportions; parts feeding means for feeding parts individually into saidfirst end portion of said tube, so that they fall through said tubespaced from one another; and means for receiving parts leaving saidsecond end portion of said tube.
 5. The apparatus described in claim 4including: a gas inlet coupled to said first end portion of said tube;and means for supplying pressured gas to said gas inlet, to create a gasflow through said tube.
 6. Apparatus for heat treating small partscomprising: an elongated tube having first and second end portions;means for holding said tube so that it extends in a primarily verticaldirection, with said first end portion above said second end portion;means for heating a center portion of said tube that lies between saidend portions; means for feeding parts into said first end portion ofsaid tube, so that they fall through said tube; means for receivingparts leaving said second end portion of said tube; and means coupled tosaid tube between said center portion of said tube and the locationwhere parts leave at said second portion of said tube, for inserting acooling gas into said tube to move with said parts as they fall towardsthe location where the parts exit from the tube.
 7. A method forsintering green cores comprising: dropping said cores individually intoa primarily vertically extending tube so that they are spaced from oneanother during their fall through the tube; and heating at least apredetermined portion of said tube.
 8. The method described in claim 7including: applying gas to the upper end of said tube at a controlledgas flow rate, whereby to regulate the passage of small parts which havea high air resistance compared to their weight.
 9. Apparatus for heattreating small parts comprising: an elongated outer tube with a centralportion; means disposed along said central portion of said outer tubefor heating the outside thereof; an inner tube disposed within andextending through said outer tube, so that said tubes are free ofcontact with one another along substantially all of said central portionof said outer tube which is heated; and means at opposite end portionsof said inner tube for receiving and delivering said parts.
 10. Theapparatus described in claim 9 wherein: said outer and inner tubesextend substantially Vertically; and including means coupled to theupper end of said inner tube for forcing a gas downwardly therethrough.11. The apparatus described in claim 9 wherein: at least one end of thespace between said central portion of said outer tube and said innertube is sealed against the axial flow of gas therethrough; and saidmeans for heating includes a plurality of separately controllableheating means spaced along said center portion of said outer tube, tocreate a plurality of separate heating zones.